A method of manufacturing an optical fiber has been disclosed in Japanese Unexamined Patent Application Publication No. 2000-233937, WO03/080522, or WO01/90010, the optical fiber being formed by inserting a columnar core glass inside a cylindrical cladding glass, followed by heating and drawing.
In the method disclosed in Japanese Unexamined Patent Application Publication No. 2000-233937, the ratio of the external diameter of the cladding is glass to the diameter of the core glass is set to 10 or more, and the dimensional error of the diameter of the core glass and that of the internal diameter of the cladding glass are each set to 5 μm or less.
In the method disclosed in WO03/080522, a core rod is coaxially disposed in a quartz glass tube having an external diameter of 100 mm or more, which is a final dimension obtained by machining, and while a space formed between the glass tube and the core rod is being collapsed, an optical fiber is drawn from the bottom end thereof.
In the method disclosed in WO01/90010, an optical fiber is produced by drawing a core rod and a cladding pipe which are coaxially disposed, wherein the cladding pipe has OH concentration of maximum 1 wt. ppm and the core rod has an optical cladding glass layer around a core glass layer, the ratio of the external diameter of the optical cladding glass layer to the diameter of the core glass layer being between 1 and 2.2, the core rod having an OH concentration of maximum 1 wt. ppm in an area close to the surface, to a depth of up to 10 μm.
A method in which a preform rod and a glass tube are sealed at one end thereof, a vacuum pump is connected to the other end, and the space formed between the preform rod and the glass tube is evacuated while the glass tube is heated so that the preform rod is overcladded with the glass tube whereby an optical fiber preform is produced, or the glass tube is collapsed on the preform rod whereby an optical fiber is drown has been disclosed in WO98/43921.
A method of obtaining an optical fiber has been disclosed in Japanese Unexamined Patent Application Publication No. 62-3034 in which a synthetic glass having a higher refractive index than that of a quartz tube is deposited inside the quartz tube, followed by drawing thereof. In the method disclosed in Japanese Unexamined Patent Application Publication No. 62-3034, one end of the quartz tube is closed, a gas in the quartz tube is then evacuated from the other end thereof for drying while the quartz tube is being heated, and subsequently, the quartz tube is filled with a halogen gas or a halogenated gas containing no hydrogen, followed by drawing for forming the optical fiber.
A method of manufacturing a large quartz glass preform has been disclosed in Japanese Unexamined Patent Application Publication No. 7-109141 in which a large quartz glass tube and a core glass rod for an optical fiber are unified together by a rod-in-tube method, the core glass rod being composed of a core portion and a part of a cladding portion, whichever transmits light (an optical cladding portion). In the method disclosed in Japanese Unexamined Patent Application Publication No. 7-109141, the large quartz glass tube has an external diameter of 50 to 300 mm, a ratio of the external diameter to the internal diameter of 1.1 to 7, a thickness of 10 mm or more, a thickness error of 2% or less, and an internal surface roughness of 20 μm or less. In addition, according to Japanese Unexamined Patent Application Publication No. 7-109141, the external diameter of the optical cladding portion must be at least 2 times the diameter of the core portion.
PCT Japanese Patent Application Translation Publication No. Tokuhyo 2002-501871 discloses a method of manufacturing an optical fiber preform. In the method, a rod is placed in a tube so as to form a circular space between the rod and the tube, one end of the tube is collapsed, the circular space is evacuated, and the other end is collapsed to form an optical fiber preform.
In general, a considerable part of light transmitted in an optical fiber penetrates into an optical cladding portion, and impurities and imperfection existing at the interface between the core and the cladding and at the optical cladding portion increase the loss of transmitting light, decrease the mechanical strength of the optical fiber, or the like, thus, seriously affecting the properties of the optical fiber. Accordingly, in general, a portion (core portion) corresponding to the core of an optical fiber is not separately formed. In particular, when a single mode optical fiber is manufactured, after a core portion and a predetermined amount of an optical cladding portion provided around the core portion are simultaneously formed so that the ratio of the diameter of the optical cladding portion to that of the core portion is 3 to 5, a remaining cladding portion is further formed therearound. Thus, the manufacturing is accomplished such that the properties of the produced optical fiber are not adversely affected even when impurities and imperfection exist in the interface between the optical cladding and the cladding provided therearound and the vicinity thereof.
In general, a refractive index profile of the core portion of the optical fiber must accurately be controlled, and hence the synthesis thereof is cost-consuming and time-consuming as compared to that of the cladding portion. However, in the case in which the core portion and the optical cladding portion are simultaneously formed as described above, the optical cladding portion can be manufactured at only a cost approximately equivalent to that of the core.